The present invention relates to ammonia synthesis gas purification and increasing the capacity of a non-Braun Purifier.TM. ammonia plant by use of the Braun Purifier.TM..
Since the advantageous use of cryogenic purification of ammonia synthesis gas as described in U.S. Pat. No. 3,442,613 (Grotz, Jr., the basis of the Braun Purifier.TM. process), it has been a goal to further advance the art of hydrogen to nitrogen ratio control and/or removal of methane and/or inerts such as argon by way of cryogenic fractionation. However, it is the overwhelming choice of non-Braun Purifier.TM. plants to install non-cryogenic separation methods of controlling hydrogen to nitrogen ratios and/or inerts reduction for ammonia synthesis gas, although the advantages of such cryogenic purification are well proven, evidenced by the use of the Braun Purifier.TM. process in about one third of the world's ammonia production capacity for high efficiency plants. The very simple and inexpensive construction of the Braun Purifier.TM. process is shown in U.S. Pat. No. 3,443,613 within the broken lines of FIG. 2 of that invention. The process of U.S. Pat. No. 3,442,613 creates an ammonia synthesis gas upstream of the Braun Purifier.TM. which preferably contains about 150% excess nitrogen over the 3:1 hydrogen to nitrogen ratio required for efficient operation of the ammonia conversion synloop. This excess nitrogen, with the rest of the CO2- and CO-free synthesis gas, is auto-refrigerated by flash expansion of rectified liquid condensate from that synthesis gas and by expander expansion of the CO2- and CO-free synthesis gas, partly cooled by autorefrigeration.
Those skilled in the art understand the exceptional operation and process of the Braun Purifier. U.S. Pat. No. 4,409,196 describes the process improvement as follows:
"It will be recognized that in the application of partial oxidation processes and of the autothermic steam reforming processes as outlined above, the employment of air as the internal oxidant is restricted by the degree that the resultant nitrogen present is acceptable in the product gas". PA1 "Thus in the usual natural gas based ammonia process, the amount of air admitted to the secondary reformer is limited to the supply of nitrogen required for the ammonia synthesis step. Also in the partial oxidation and autothermic reforming operations, recourse to at least partial supply of the oxidant in the form of substantially pure oxygen is usually necessary, except when the process is to be used only to produce a low grade fuel gas. The necessity for the supply of substantially pure oxygen means that an air separation plant must be provided. The additional capital and running costs incurred thereby results in such processes appearing less attractive as a means of producing hydrogen rich gases except when the feed hydrocarbon is very cheap or complete flexibility of feedstock source is desired". PA1 "One exception to this restriction is in the Braun "Purified" Process for the Manufacture of ammonia disclosed in U.S. Pat. No. 3,442,613. In the process disclosed a synthesis gas stream is obtained by primary reforming methane or other hydrocarbon with steam followed by a secondary reforming in which air is present in an amount to provide a stoichiometric excess of nitrogen from 2 to 150 mole percent based upon that needed for the synthesis gas. The excess nitrogen is condensed downstream of the reformer." (col. 2, II. 21-51)
The Braun Purifier process is the only commercially important process for preparation of synthesis gas with excess nitrogen removed by the simple combination of gas expansion, auto-refrigeration and cryogenic fractionation. That process obtains the required 3:1 hydrogen to nitrogen ratio most efficient for use in the synthesis loop with additional benefit of reducing purge gas volume by obtaining extremely low levels of methane and inerts from the Braun Purifier step. In the past, the Braun Purifier process has only been used in so-called "grass roots" plants, i.e., plants for which the large and expensive equipment for primary and secondary reformers, shift reactor, carbon dioxide removal, methanation, compression and heat recovery are integrated in a design with the autorefrigeration and cryogenic fractionation. It is unknown to have tried expansion of an installed Braun Purifier process without the assistance of the owners of that process. No substantial expansion of such an installed process has previously been known without changing the multi-stream platefin exchangers and at least some portion of the cryogenic fractionation column and/or condenser, a process requiring the expertise of the owners of the process, presently Brown & Root, Inc.
U.S. Pat. No. 4,681,745 describes production of ammonia by the sequence of steam hydrocarbon primary reforming, secondary reforming with air, carbon monoxide shift conversion, carbon oxides removal and catalytic ammonia synthesis is improved by using oxygen-enriched air at secondary reforming and/or by operating the reforming steps so that 5-15% by carbon atoms of the starting hydrocarbon is not reformed but is purged from the synthesis. The oxygen enriched air can be the by-product of a simple air separation plant producing nitrogen, and the nitrogen can be used to aid start-up or shut-down of the process or to keep the process plant in a hot stand-by condition.
U.S. Pat. No. 4,383,982 describes an ammonia production process whereby a hydrocarbon steam mixture is preheated and reacted in an adiabatic catalyst bed, the resulting methane-containing gas is reacted with air to introduce more than 1 N(2) per 3H(2) and the purified gas is passed to ammonia synthesis in admixture with a hydrogen-rich stream separated at 80 from reacted synthesis gas, and the rate of flow of that stream is controlled so that the H(2):dN(2) ratio of the gas entering the synthesis catalyst is in the range 1.0 to 2.5. Preferably methane is purged at 86 to the extent of 5-15% of the hydrocarbon feedstock.
U.S. Pat. No. 4,296,085 describes a process to produce ammonia from a hydrocarbon feedstock, involving basically the following steps: dividing the feedstock into two fractions, subjecting the first fraction to a primary steam reforming reaction, at high pressure and moderate temperature, combining the effluent from the primary reforming with the second fraction of the feedstock, and subjecting the mixture thereof to a secondary adiabatic reforming reaction with an amount of air in excess to that needed for ammonia synthesis, subjecting the synthesis gas produced to a CO shift conversion reaction, and then to CO(2) removal by solvent scrubbing, while the gas released by pressure letdown of said solvent is preferably recycled back upstream of the secondary reforming, methanation of the residual carbon oxides, removing the excess nitrogen present in the gas by cryogenic separation, compressing and feeding the final synthesis gas into an ammonia synthesis loop, recycling the purge gas from said ammonia synthesis loop to upstream the cryogenic separation.
U.S. Pat. No. 4,925,456 describes a plurality of double pipe heat exchangers which are used for primary reforming in a combined primary and secondary reforming process and apparatus.
U.S. Pat. No. 4,780,298 describes an ammonia production process in which excess nitrogen and traces of carbon oxides are removed from raw ammonia synthesis gas firstly by application of partial condensation and secondly by application of washing action provided by carbon-oxides-free liquefied gas, rich in nitrogen, which is derived from a cryogenic process used preferentially for separation of hydrogen from the ammonia synthesis loop purge gas as produced in processes which use excess of nitrogen above stoichiometric requirements in the circulating gas in the synthesis loop.
U.S. Pat. No. 4,409,196 describes a process for producing a gas stream for ammonia synthesis in which a gas stream containing hydrogen and nitrogen in excess of ammonia synthesis requirements, e.g. obtained by partial oxidation of natural gas, coal or oil, is treated to remove other component gases and thereafter subjected to a separation stage, e.g. in a cryogenic separator, to separate a hydrogen-nitrogen stream having the desired hydrogen:nitrogen ratio which is injected into the reactor for ammonia synthesis, and a waste nitrogen stream which may be utilized in power generation or washing stages.
U.S. Pat. No. 4,613,492 describes a process for the production of ammonia wherein excess nitrogen is fed to the secondary reformer and a cryogenic unit is employed to obtain a nitrogen-rich stream which is recycled at least in part to the cryogenic unit.
U.S. Pat. No. 4,699,772 describes a process for preparing ammonia from hydrogen and nitrogen wherein the synthesis gas mixture is produced by partial oxidation, in the presence of a suitable catalyst, at a pressure of from 35 to 150 bar and temperatures of from 850 deg. -1200 deg. C. at the exit of the partial oxidation zone, followed by removal of the carbon oxides and water from the gaseous effluent of the partial oxidation zone. The air used for the catalytic partial oxidation is supplied in such a quantity that the molar ratio of hydrogen to nitrogen in the synthesis gas is between 2.5 and 3 to 1 and is enriched with such a quantity of oxygen that the total quantity of oxygen is sufficient to effect the required degree of hydrocarbon conversion.
It will be seen that the prior art has striven mightily to meet the following problem without benefit of the Braun Purifier process. If a hydrogen to nitrogen ratio of 3:1 is to be achieved far downstream at the ammonia synthesis loop without nitrogen separation or autothermal reforming, the heat input required at the secondary reforming step is far from adequate if non-enriched air is used for the high pressure oxidation at that stage. The end result is that (1) nitrogen removal must be achieved in some manner or (2) heat transfer to the secondary reformer must be delivered in some way other than directly by non-enriched air oxidation.
The method of obtaining projects for which an ammonia synthesis design will be paid for is a complex one, but at the same time is quite simple. A buying decision must balance technical superiority with price. Since 1990, the Braun Purifier process has been installed in four ammonia plants in China, each with capacities of over 1500 metric tons per day. The process is substantially the same as that shown in U.S. Pat. No. 3,442,613, which is incorporated herein. Obviously, the overwhelming tendency of this art is to resist change as being a risk to operational efficiency and safety.
U.S. Pat. Nos. 4,409,196 and 4,780,298 use a liquid nitrogen wash stream generated external to the synthesis gas loop to reduce excess nitrogen. U.S. Pat. Nos. 4,699,772 and 4,681,745 reduce excess nitrogen by using enriched air in the secondary reforming step, again requiring an external refrigeration and separation process to produce synthesis gas with a proper hydrogen to nitrogen ratio. U.S. Pat. Nos. 4,613,492 and 4,296,085 describe using a recycle stream of ammonia synthesis loop purge gas to adjust that ratio. U.S. Pat. No. 4,383,982 operates a synthesis loop at a hydrogen to nitrogen ratio less than 3.0 with recycle streams and purge gas stream separation. U.S. Pat. No. 4,925,456 is exemplary of heat transfer equipment attempting to transfer heat from the secondary reforming step to the primary reforming step to achieve an autothermal reforming. Such autothermal reforming has been found to fall short of the overall process heating requirements for making a synthesis gas of proper hydrogen to nitrogen ratio without having excess nitrogen.
Although the prior art indicates certain advantages of the manipulation of excess nitrogen in the synthesis gas stream, the advantages are not such that they indicate an organized direction to the skilled person.